A private branch exchange (PBX) is a telephone exchange that may serve a particular business or office, as opposed to an exchange that a common carrier or telephone company may operate for the general public. A PBX, for example, may connect the internal telephones of a private organization and also may connect the internal telephones to a public switched telephone network (PSTN) via trunk lines. A Central exchange (Centrex) is a PBX-like service providing switching at a central office (e.g., at the telephone company) rather than at the private organization's premises. In this case, the telephone company may own and manage the communications equipment (e.g., a switch) and software to implement the Centrex service. The telephone company may sell the services to the private organization (e.g., the customer).